The subject matter described herein relates to a catalyst system which facilitates the reduction of nitrogen oxides in vehicle exhaust gases, and more particularly to a catalyst system which reduces nitrous oxide (N2O) emissions from vehicle exhaust gases.
Environmental regulations pertaining to the reduction of emissions from vehicular engines are becoming increasingly stringent, requiring new designs to address the challenges of producing ever cleaner vehicles. Catalysts have long been used in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxide (NOx) pollutants into more environmentally benign gases such as carbon dioxide, water vapor, and nitrogen. It has long been known that so-called three-way catalysts (TWC) can be used to simultaneously convert carbon monoxide, hydrocarbons, and nitrogen oxides into more environmentally benign gases. Generally, such TWCs include a plurality or multiple precious metals such as platinum group metals (PGMs). For example, catalysts comprised of combinations of rhodium (Rh) and platinum (Pt) or palladium (Pd) have been used in TWC systems. Generally, it is believed that Pd is more effective to convert CO and HC, while Rh is more effective to reduce NOx compounds.
Oxides of nitrogen are of an increasing environmental concern. For example, the National Research Council in a 2010 study estimated that N2O molecules released into the atmosphere stay intact for an average of 120 years, and the environmental impact of one pound of N2O on atmospheric warming is over 300 times that of one pound of carbon dioxide. Increasingly stringent fuel economy standards exacerbate the production of NOx in vehicles.
Thus, it is preferable to operate vehicle engines under lean conditions, i.e., conditions where the air/fuel ratio is greater than stoichiometric to improve fuel efficiency and lower CO2 emissions. However, while lean operation improves fuel economy, such operation increase the difficulty of treating and removing NOx gases from vehicle exhaust systems. This has resulted in vehicle manufacturers developing so-called lean NOx traps which add to the complexity and cost of exhaust systems in an attempt to capture and reduce as much of the NOx as possible.
Other conditions encountered during normal operation of motor vehicles also result in the production of excess NOx gases. For example, during cold start-up of engines, the effectiveness and efficiency of the catalysts is reduced until the catalysts reach their light-off temperature. During vehicle acceleration, the increased flow of exhaust gases containing NOx and CO result in incomplete reduction of NOx compounds to N2 and cause additional N2O to be generated in the catalytic converter.
Accordingly, a need remains in this art to provide more efficient catalyst systems that are able to address and control emissions of pollutant gases over a wide range of operating conditions.